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About assembly : Basic-use-of-immediates-vs-square-brackets-in-YASMNASM-x86-assembly

Question Detail

Suppose I have the following declared:

section .bss
buffer    resb     1

And these instructions follow in section .text:

mov    al, 5                    ; mov-immediate
mov    [buffer], al             ; store
mov    bl, [buffer]             ; load
mov    cl, buffer               ; mov-immediate?

Am I correct in understanding that bl will contain the value 5, and cl will contain the memory address of the variable buffer?

I am confused about the differences between

  • moving an immediate into a register,
  • moving a register into an immediate (what goes in, the data or the address?) and
  • moving an immediate into a register without the brackets
    • For example, mov cl, buffer vs mov cl, [buffer]

UPDATE: After reading the responses, I suppose the following summary is accurate:

  • mov edi, array puts the memory address of the zeroth array index in edi. i.e. the label address.
  • mov byte [edi], 3 puts the VALUE 3 into the zeroth index of the array
  • after add edi, 3, edi now contains the memory address of the 3rd index of the array
  • mov al, [array] loads the DATA at the zeroth index into al.
  • mov al, [array+3] loads the DATA at the third index into al.
  • mov [al], [array] is invalid because x86 can’t encode 2 explicit memory operands, and because al is only 8 bits and can’t be used even in a 16-bit addressing mode. Referencing the contents of a memory location. (x86 addressing modes)
  • mov array, 3 is invalid, because you can’t say “Hey, I don’t like the offset at which array is stored, so I’ll call it 3″. An immediate can only be a source operand.
  • mov byte [array], 3 puts the value 3 into the zeroth index (first byte) of the array. The byte specifier is needed to avoid ambiguity between byte/word/dword for instructions with memory, immediate operands. That would be an assemble-time error (ambiguous operand size) otherwise.

Please mention if any of these is false. (editor’s note: I fixed syntax errors / ambiguities so the valid ones actually are valid NASM syntax. And linked other Q&As for details)

Question Answer

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